influence the separating efficiency of a continuous

(i) What is the q line and how can it influence the separating efficiency of a continuous

distillation column? [5 marks]

(ii) A continuous fractionation column, operating at atmospheric pressure, is to be

designed to separate a mixture containing 15.67% CS2 and 84.33% CCl4 into an

overhead product of 91% CS2 and a waste of 97.3% CCl4 (all in mass percent). Feed

enters at 290K with a specific heat of 1.7 kJ/kg K and a boiling point of 336K.

Assuming a plate efficiency of 70% and a reflux of 3.16 kmol per kmol of product,

determine the number of plates required using the vapour liquid equilibrium data sheet

provided in Figure 1 (see page 5). [20 marks]

The latent heat of CS2 and CCl4 is 25,900 kJ/kmol.

  1. (i) Discuss the advantages and disadvantages of sieve plates in a plate column. How does

the vapour and liquid flow affect the operating limits of sieve trays? [5 marks]

(ii) A mixture of water and ethanol containing a 0.16 mole fraction of ethanol is

continuously distilled in a plate fractionating column to give a product containing a

0.77 mole fraction of ethanol and a waste of 0.02 mole fraction of ethanol. It is

proposed to withdraw some of the liquor via a side stream which will contain 25% of

the ethanol in the entering stream at a mole fraction of ethanol of 0.5.

Using the vapour liquid equilibrium curve for the ethanol-water system given in Figure

2 (see page 6), determine the number of theoretical plates required and the plate from

which the side stream should be withdrawn if the feed is a liquor at its boiling point

and a reflux ratio of 2 is used. [20 marks]

 

(i) What criteria should be considered when making a choice of solvent for liquid-liquid

extraction? [5 marks]

It is proposed to reduce the concentration of acetaldehyde in aqueous solution from 50%

to 5% by mass, by extraction with solvent, S, at 293K. If a counter current multistage

process is adopted and 0.025 kg/s of the solution is treated with an equal quantity of the

solvent, determine:

(ii) The number of theoretical stages required. [15 marks]

(iii) The mass flowrate and concentration of the extract of the first stage. [5 marks]

The equilibrium relationship for this system at 293K is given in Figure 3 (see page 7).

 

  1. Answer either section A or Section B

Section A

(i). Describe the three basic configurations of single effect evaporators. What are their

advantages and disadvantages in relation to dealing with different liquid types?

[5 marks]

(ii). Sodium hydroxide solution is to be concentrated from 10% to 30% by weight

using a single effect evaporator operating at atmospheric pressure. The liquid

circulates through the tube side of a calandria consisting of 2 m long tubes, 32 mm

OD and 28 mm ID. Dry saturated stream is circulated through the shell side of the

calandria at 394 K. If the overall heat transfer coefficient is 1.75 kW m-2

how

many tubes will the calandria require to concentrate 0.075 kg s-1

of dilute

solution? [20 marks]

The specific heat of the solution can be assumed to be constant at 4.18 kJ kg-1 K

-1

.

The latent heat of evaporation is 2270 kJ kg-1

and the boiling point rise is 15K.

The feed temperature is 303 K

Section B

A non-volatile oil contains 2.2 mol % of a volatile contaminant B which is reduced to

0.04 mole per cent. Live steam is used in the ratio 5 kmol of steam to strip 100 kmol

of B from the oil. The operation is carried out in a counter current flow in a packed

column 3 m high. If the vapour-liquid equilibrium relationship for B is given by

ye=40x where ye and x are mole fractions in the vapour and liquid respectively,

(i) Calculate the flow of B leaving in the volatile oil. [5 marks]

(ii) Calculate the operating line for these conditions. [5 marks]

(iii) Construct the equilibrium line consisting of at least 6 points and plot the

operating line. [10 marks]

(iv) Using the diagram constructed in part (iii), calculate the number of ideal

stages and hence the height of transfer unit for the packing. [5 marks]